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Discussion Starter #1
I have a new "502" in my `70 Chevelle, with a standard transmission. I have a new Optima battery, and 100 amp power master alt. I wired the car with instructions from Mad-electronics, with a terminal block on the firewall, and used relays for the headlights, ignition, fuel pump, and fans. the fans relays and headlights are powered off the horn buss bar. the rest on the terminal block on the fire wall. The voltage sensing wire is also hooked to the ternimal block on the firewall. When I first start the car the volt gague reads 14 volts. When the cooling fans turn on (which take 50 amps) the volt gague drops to 13 and sometimes lower. If I run the stereo, air conditioner, and headlights it will go to 12.I took the car to the local alt shop and had the system checked, they told me that the alt didnt put out enough amperage at idle and would order a new one that produces 140 amps and, recomended I run #2 welding leads from the new alt all the way to the battery, I now have 8 gage wire to the bus bar and the orignal I think 12 gage from there to the battery. The stereo doesnt have an external amp or big wolfers, just a straight c.d. player. Shouldnt this alt handle this amount of accesories? What if I ran the sensing wire to the horn relay where the fans pull all thoes amps, and how many amps can a 100 amp alt produce continually without harm? What do you think about the huge welding leads? This is a longtime business in the area with a good reputation, that recomended these changes. I am not so sure... By the way when I returned from the shop I removed my battery and put it on the charger, the meter went to 6 amps, I dont think it had a full charge, the shop is about a 20 mile round trip. Any suggestions appreciated..
 

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I don't know what fans you are running but I am in the process of loading a Derale dual 13 inch setup (Rated 4000 CFM) in my car. They day 140 amps NO LESS. I am not running yet so I dont know what my ultimate need will be but everyone I have talked to...Engineers with experience designing DC circuits included) say big alternator output is a necessity. The issue is that you cant just add up the load presented by the devices in the car and use that number to size the alternator. If your batter is below total charge then it is going to tap off some of the juice from the alternator. If after adding all the loads up you only have a few amps to spare you get in trouble when the battery is low.

Also the amount of alternator output at a give RPM becomes important. At low engine speeds like in town your 100 amp alternator (for example) might not be generating even say 80 amps. Just when the stereo is up and running, the fans are on and the A/C is on guess what the alternator is down at lower than max output.

In summary if they say 140 amps at the electrical shop they likely are right. I spoke to Derale before I bought the fan and 140 is the number they came up with for their product. You don't mention what brand you have but the Derale unit wants 50 amps just like you mention for the requirement of your fan.
 

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Discussion Starter #3
My fans are also Derale,dual units. And when they come on the Alt knows it.. They will also blow your hat off, I have dual Flex-a-lites on my `68 and they are barely adiquate, these Derale fans work verry well, they just use a lot of "juice". Be sure you use 2 relays, one for each side, I wired mine up as per instructions and a week later was adjusting the carb and noticed only one side was working, after talking to Derale they told me to run one fan on the relay that came with the units and buy another relay for the other side. What size wire are you running from the alt to the buss bar? thanks for the reply...Stan
 

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I don't fully understand the wiring. Does the wire from the alternator go to the horn relay and then from there to the new terminal block on the firewall? Or are there seperate wires from the alternator to the horn relay and the firewall?

The wires from the battery to horn relay and from the alternator to the horn relay are both too small. I'd run as a minimum #4 for those runs. You need to use the proper size wire for the alternator output from the alternator to the battery and to the connection of any high output accessories. Then, when you branch off to smaller wires such as the origional dash power feed or the light relay power you need to put a fusible link on the wire to protect it. Personally, I think #2 is a little overkill, even for a 140A alternator.

If the wiring goes from the alternator to the horn relay to the firewall terminal then changing the remote sensing wire won't help because it is already compensating for the voltage drop in the wire from the alternator to the horn relay. If there are 2 wires from the alternator to each place then it may.

What happens when you raise the rpm's? If it only drops voltage at idle it may be OK if you don't idle too much when driving it. Remember, on average the alternator has to charge the battery more than the accessories discharge the battery.

Peter
 

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Discussion Starter #5
The wire from the alt is 8 gauge to the block on the firewall, from there its 8 gague to the buss bar on the horn relay, its new wire, then its the original wire, not sure but I think its 10 or 12 gague? to the battery, with a fusible link. The sensing wire goes to the block on the firewall.
 

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OK, what I would do is go with #4 from the alternator to the horn relay and #4 from the horn relay to the battery. Your fan must connect to this #4 wire. If the horn relay isn't heavy enough to properly connect these wires then put a new terminal block there that is. Leave the #8 from the horn relay to the firewall. The run from the alternator to the firewall and back to the front is alfully long to be running those fans with, especially with #8 wire. You want the highest current path (the alternator powering the fans in this case) to be as short as possible.

You can likely leave the remote sense wire where it is. The only reason you'd need to move it is if the battery is being overcharged which likely won't happen but you should check to be sure.

Once you change the wiring, then see how the alternator is doing. If it still isn't keeping up enough then you'll have to upgrade it too.

Peter
 
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